1. Field of the Invention
The invention disclosed in this specification relates to a structure of a thin film semiconductor device such as a thin film transistor or the like, and a method for fabricating the same.
2. Description of the Prior Art
There is known a structure for obtaining a display device which has a high display function by using a thin film transistor (TFT) in a liquid crystal display, the display substituting a cathode ray tube. This display is referred to as an active matrix type liquid crystal display device. This active matrix type liquid crystal display device is a display in which a thin film transistor is arranged in each of the pixel electrodes arranged in matrix to provide a high function display. To heighten the display function, the characteristics of the thin film transistor is required to be set to as high as possible.
The thin film transistor used in an active matrix type liquid crystal display device has a problem in that the thin film transistor is required to be formed on a glass substrate. In other words, to use the glass substrate as the substrate, there is a problem in that the substrate is limited in the fabrication process. Not only thin film transistors but also semiconductors needs to be heated to a high temperature (for example, of 800 to 1000° C.) out of the necessity of diffusing impurity into silicon, activating impurity in silicon, and improving the crystallinity of silicon. However, the temperature that can be applied to the glass substrate is generally about 600° C., and various new techniques are required to fabricate a high performance semiconductor device at a temperature level below this. For example, there are such techniques as a technique for irradiating an amorphous silicon film with laser light to crystallize the amorphous silicon film, and a technique for using the laser light irradiation for the diffusion and activation of the impurity. Since the technique for laser light irradiation causes an extremely small thermal damage to the glass substrate, this is an extremely useful technique when the low productivity is permitted.
FIG. 2 is a schematic sectional view of a conventionally known thin film transistor (generally referred to as TFT). What is shown in FIG. 2 is a thin film transistor which functions to prevent the intrusion of impurity into the active layer from the glass substrate. The active layer comprises a source area 203, a channel formation area 204 and a drain area 205. Then, as a gate insulating film 200, a silicon oxide film or a silicon nitride film is formed. A gate electrode 206 comprises a metal and semiconductors. Further, the whole element is covered with an interlayer insulating film 207 which comprises an appropriate insulator such as a silicon oxide film or the like. Further, a source electrode 208 is taken out from the source area 203 while a drain electrode 209 is taken out from the drain area 205.
The active layer comprising the source area 203, the drain area 205 and the channel formation area 204 is formed of crystalline silicon. As the crystalline silicon film, a silicon film formed of the amorphous silicon film crystallized by the laser light irradiation is used. However, there is no technique available for forming a single crystal silicon on the glass substrate. Although the film thus formed has crystallinity, the film has a quality in which a large amount of defects and levels are present. Although the film thus formed has crystallinity, the film has a quality in which defects and levels are present. To reduce the defects and levels in the silicon film, a method for neutralizing a dangling bond (unpaired connectors) of silicon which causes defects and levels by using a hydrogen atom. This holds true of a case in which the active layer is not crystalline silicon and is formed of amorphous silicon.
In this manner, in the silicon semiconductor film formed on the glass substrate, the silicon semiconductor film needs to contain hydrogen. However, when an attempt is made to cause the active layer formed of the silicon semiconductor to contain hydrogen, there is a problem in that hydrogen is diffused into the gate insulating film from the active layer.
On the other hand, in the structure shown in FIG. 2, it is not extremely favorable that mobile ions exist in the gate insulating film, the threshold value varies, and a hysteresis is generated in the C-V characteristics. Consequently, containing hydrogen in the active layer is a useful method on the one hand, it is a disadvantageous method on the other in that hydrogen is diffused in the gate insulating film.